.back_top{
  position: fixed;
  width: 40px;
  height: 40px;
  border-radius: 50%;
  color: pink;
  display: flex;
  align-items: center;
  justify-content: center;
  box-shadow: 0 0 6px rgba(0,0,0,.12);
  cursor: pointer;
  z-index: 5;
}
.back_top:hover{
  background-color: pink;
}